1. Field of the Invention
The present invention relates to an optical transmitter apparatus for generating and transmitting a phase-modulated optical signal, and particularly relates to an optical transmitter apparatus for generating and transmitting an optical DQPSK signal.
2. Description of the Related Art
Phase modulation has been in practical use as one of the technologies for transmitting signals in an optical transmission system. In the phase modulation, data is transmitted by controlling the phase of a carrier wave in accordance with the transmission data. For example, in QPSK (Quadrature Phase Shift Keying) modulation, “θ”, “θ+π/2”, “θ+π”, and “θ+3π/2” are assigned respectively to each symbol “00”, “10”, “11”, and “01”, consisting of two bit data. Here, the “θ” is an arbitrary phase. A receiver device recovers the transmission data by detecting the phase of the received signal.
DQPSK (Differential Quadrature Phase Shift Keying) is also known as a technology, which realizes the QPSK receiver apparatus, with relative ease. In the DQPSK, the difference between two successive symbols is assigned with the corresponding phase (θ, π/2, π, 3π/2). Therefore, the receiver device can recover the transmission data by detecting the phase difference between the two successive symbols.
FIG. 1 is a diagram describing the configuration of a DQPSK transmitter apparatus. In this drawing, only necessary configuration is shown in order to explain the operation principle.
An optical DQPSK modulator 100 is a Mach-Zehnder modulator, and is provided with optical continuous wave (CW) generated by an optical source 111. The optical CW is split by an optical splitter, and is guided to a first arm and a second arm. In the first arm, a phase modulator 101 is provided, and in the second arm, a phase modulator 102 and a phase shifter 103 are provided. The phase modulators 101 and 102 are driven by driving signals data 1 and data 2, respectively. Here, the driving signals data 1 and data 2 are generated by a data generator unit 112 comprising a DQPSK pre-coder. The phase shifter 103 provides phase difference π/2 between the first and the second arms. A DQPSK signal is generated by combining a pair of optical signals output from the phase modulators 101 and 102.
FIG. 2 is a diagram explaining the operation of the DQPSK transmitter apparatus. Here, assume that the phase of the optical continuous wave is zero.
The phase of the optical signal output from the phase modulator 101 becomes “0” or “π” in accordance with the driving signal data 1. On the other hand, the second arm comprises a phase shifter 103 providing the phase difference of π/2. Therefore, the phase of the optical signal output from the phase modulator 102 is “π/2” or “3π/2” in accordance with the driving signal data 2. Then, “π/4”, “3π/4”, “5π/4”, or “7π/4” is assigned to each symbol with two bit information (00, 10, 11, 01). The receiver device recovers the transmission data by detecting the phase difference between successive two symbols.
Detailed description of the configuration and the operation of the optical DQPSK transmitter apparatus is provided in, for example, a Patent Document 1 (US2004/0081470 or WO2002/051041). In addition, a Patent Document 2 (US2004/0028418) describes a configuration performing feedback control of the DC bias of a Mach-Zehnder modulator.
In order to improve the quality of communications, which employ the DQPSK (especially, high speed data communication with several ten Gb/s), the phase of the optical signal output from the optical DQPSK transmitter apparatus has to be accurately controlled. However, the DQPSK is a technology under development, and still has a number of problems to be solved or improved.
If the amplitude of the driving signals data 1 and data 2 is not appropriately adjusted, for example, the phase of the DQPSK signal deviates from a desired phase. In an example shown in FIG. 3, the amplitude of the driving signal data 1 is smaller than a prescribed amplitude. In such a case, the intensity of the generated DQPSK signal also deviates. Note that the amplitude of the driving signal may deviate due to the aging degradation in the electrical drive system, variations in characteristics of electronic components or optical components, or thermal change.
If the DC bias of the phase modulators 101 and 102 or the amount of phase shift of the phase shifter 103 is not appropriate, the phase of the DQPSK signal deviates from the desired phase.
These problems can occur in the QPSK as well as in the DQPSK.